Glyphosate - A Review
by Bob HartzlerNovember 13, 2001
- In recent years, a large number of herbicides based on the active
ingredient glyphosate have been introduced. Allclaim
to be as good, or better, than the original - Roundup.The ingredient statements on the label provides
little help in differentiating the products since the contents are broken down simply as
active and inert or other ingredients.This article will discuss how the contents
of the glyphosate products may vary and factors that influence the performance of
glyphosate.

The active ingredient, glyphosate, is the
compound that actually kills weeds.The
Roundup Ultra label states that the active ingredient is Glyphosate,
N-(phosphonomethyl)glycine, in the form of its isopropylamine salt.The term glyphosate is the
common name of the chemical, whereas N-(phosphonomethyl)glycine is the
chemical name that provides information about the actual chemical structure of the
herbicide.Regardless of the brandyou purchase, the active ingredient for all
glyphosate products is exactly the same.

Glyphosate is a substituted amino acid that
interferes with amino acid synthesis by inhibiting the EPSPS enzyme.This enzyme is involved in the synthesis of
several amino acids, the building blocks of proteins.Several factors contribute to the effectiveness of glyphosate: 1) The EPSPS enzyme
is a part of an important metabolic pathway in all plants.Disruption of this pathway is normally fatal to the plant; 2) Glyphosate binds very
tightly to the EPSPS enzyme. Thus, once the herbicide reaches the target site, the enzyme
essentially is nonfunctional;3) Plants are
inefficient at metabolizing glyphosate, thus the molecule remains intact within the plant
until it reaches the target site; and 4) Glyphosate does not cause a rapid disruption of
plant tissue.This allows the herbicide to be
translocated throughout the plant, providing a more effective kill than herbicides that
rapidly disrupt plant tissues.

Glyphosate is a type of chemical known as a
weak acid.Weak acidscan donate a hydrogen ion to other compounds
(Figure 1).When glyphosate is formulated
into a commercial product, the hydrogen ion on the parent weak acid is replaced with a
different salt (ion).The salt itself does
not have herbicidal properties, but results in a product that is easier to handle, mixes
better with other agricultural chemicals, and/or is more effective than the parent weak
acid.

All glyphosate products except Touchdown
contain the isopropylamine salt (IPA) of glyphosate.Touchdown IQ contains the diammonium salt (DAM) of glyphosate.The particular salt formulationdoes not significantly affect the performance of
glyphosate.However, some salts may have
phytotoxic properties.The trimethylsulfonium
salt (TMS) used in the original Touchdown formulationcaused
localized burning of leaves.While this
damage was insignificant in comparison to the herbicidal properties of glyphosate, the
salt did cause minor injury to the foliage of Roundup Ready crops.The new Touchdown IQ formulation does not have
this characteristic.

The label of most glyphosate products
reports the concentration (lbs/gal) in terms of both active ingredient (a.i.) and acid
equivalent (a.e.).When calculating the
quantity of active ingredient in a product, the weight of both the parent acid of
glyphosate and the weight of the salt used to formulate the product is considered.The quantity of acid equivalent reported on the
label only takes into the amount of parent acid in the product, the weight of the salt
formulated with the product is not considered.Thus,
acid equivalent is a better measure of the relative strength of glyphosate products since
the salt does not contribute to herbicidal activity.When comparing Touchdown to other glyphosate products, it is important to compare
acid equivalent rates rather than active ingredient since different salts are used in
these products (Table 1).

Table 1.Comparison
of glyphosate products.

Product

Salt

Concentration

Equivalent
rates of product

Active
Ingredient (a.i.)

Acid
Equivalent (a.e.)

Roundup UltraMax

IPA

5 lbs/gal

3.7 lbs/gal

26 oz

Roundup Ultra, Glyphomax Plus,
Glyphos, Roundup, etc.

IPA

4 lbs/gal

3 lbs/gal

32 oz

Roundup UltraDry

IPA

71.4%

64.9%

0.75 lbs

Touchdown IQ

DAM

3.6 lbs/gal

3 lbs/gal

32 oz

Touchdown 5

TMS

5 lbs/gal

3.4 lbs/gal

28 oz

The inert ingredients make up approximately
50 to 75% of most glyphosate products. These materials serve a variety of important
functions, such as improving the handling characteristics and stability of the product,
enhancing compatibility, and most importantly, improving retention and absorption of the
herbicide by plants.Any differences in
performance of glyphosate products is likely to be caused by the inert
ingredients used in the product, rather than the salt of glyphosate used in the formulated
product.The specific inert ingredients
material used inproducts are proprietary
information, and herbicide manufacturers consider them to trade secrets.

In terms of herbicidal activity,
surfactacts are the most important component of the inert ingredients.The types of surfactant formulated with glyphosate
have a significant effect on the performance of the specific product.However, manufacturers invest significant
resources in developing formulations before the products are introduced to the market.
Differences in performance among glyphosate products attributed to the surfactants are
relatively small when compared to the other factors that influence field performance. If
there were significant differences, it is likely that the herbicide rates recommended on
the label would vary to account for these differences.However, all products generally recommend equivalent rates of glyphosate for
similar uses.

Consistent performance is one of the
primary reasons for the popularity of glyphosate.However,
as with any other herbicide, many factors can lead to variable control with glyphosate.This article will review factors that influence
the activity of glyphosate and how the herbicide can be managed to minimize fluctuations
in performance.

The primary difference among the many available glyphosate products is the surfactant
mixture found in the formulated product.Surfactants
enhance the retention and absorption of glyphosate by plants contacted by the spray
solution.Although the blend and amount of
surfactants vary among the many glyphosate brands, performance of these products is
similar under most conditions.In recent
years Iowa State University has conducted numerous experiments to determine if glyphosate
products perform differently in the field.Eleven
field trials were conducted in 2001 in which the performance of Roundup UltraMax was
compared to Touchdown IQ, Glyphos Gold, Glyphomax Plus or other brands of glyphosate.In 91% of the comparisons there were no
differences in performance when comparing products at equivalent rates of active
equivalent with recommended additives (Table 2).All
glyphosate products performed equally on foxtail, velvetleaf and waterhemp in the eleven
experiments.Roundup UltraMax provided better
control of common lambsquarter than the other formulation in two of nine situations.On other species, the generic formulations
performed similarly to Roundup UltraMax in six of eight comparisons, with one situation
where the generic performed better and one where it performed worse than the Monsanto
brand.No readily apparent differences in
the rate of kill was observed with different
glyphosate brands. These data suggest that performance differences among glyphosate
brands are small and should not be a major criteria in product selection.

Table 2.Summary
of performance comparisons of different glyphosate formulations applied at equivalent
rates in ISU field trials
during 2001 growing season1.

Foxtail

Velvetleaf

Lambsquarter

Waterhemp

Other

Same

Different

Same

Different

Same

Different

Same

Different

Same

Different

12

0

10

0

7

2

7

0

6

2

1Comparisons made between equivalent acid equivalent rates
of glyphosate with recommended spray additives.

Spray
additivesAll brands of
glyphosate recommend the addition of AMS under certain conditions (see water quality), but
recommendations for surfactants vary widely among glyphosate products.The Roundup UltraMAX label states Do not add
surfactants, additives containing surfactants, buffering agents or pH adjusting agents to
the spray solution when Roundup UltraMAX is the only pesticide used unless otherwise
directed.On the other hand,
DowAgroSciences recommends the addition of surfactant with Glyphomax but not with
Glyphomax Plus.The differences in
recommendations are due to the amount and type of surfactant included in the formulated
product.Monsanto believes that Roundup
UltraMAX has the optimum blend of additives to maximize performance, and thus there would
be no benefit to including additional surfactants in the spray tank.Other manufacturers have chosen to give the user
flexibility in selecting additives.

Two questionsfrequently arise
concerning additive use with glyphosate products:1)
Is there a benefit to using additional surfactants with brands that do not recommend
them?, and 2) What surfactant is best?The
answer to the first question is probably not.Occasionally
the performance of glyphosate products not recommending a surfactant might be enhanced by
including additional surfactant.However, the
inability to predict when this will occur results in the only person benefiting from this
practice in the majority of situations is the person selling the surfactant.

Selecting the optimum surfactant is complicated by the fact that
manufacturers of surfactants and other spray additives are not required to provide
information on the products active ingredients.Thus it is impossible to compare the numerous products available for this use.The risk of obtaining a poor quality
surfactant can be minimized by obtaining products with a high concentration of active
ingredients, avoiding products making unrealistic claims, and purchasing spray additives
from the seller of the herbicide it is intended to be used with.

Water quality Whether the water used as the carrier
for glyphosate comes from a well or a rural water association, it may containlarge amounts of dissolved salts.Water hardness is a measure of how much salt is
contained in the water.The harder the water,
the higher the salt concentration.Salts
dissolved in water may reduce the effectiveness of glyphosate, particularly calcium and
magnesium salts.These salts have a positive
charge and may associate with the negatively-charged glyphosate molecule, replacing the
isopropylamine or diammonium salts found in the formulated glyphosate product.Glyphosate that is bound with calcium or magnesium
salts is less readily absorbed by plants than the form of glyphosate present in the
product container.Thus, the presence
of calcium and magnesium salts in the carrier can result in a reduction in glyphosate
activity.

Although specific recommendations vary, all
products containing glyphosate labels recommend the addition of ammonium sulfate (AMS).The role of AMS as an additive with glyphosate is
considerably different than the function of the non-ionic surfactants or crop oil
concentrates (COC) commonly used with postemergence herbicides.Whereas surfactants and COCs are active
primarily on the leaf surface and improve absorption of the herbicide into plants, AMS is
primarily active within the spray tank.

The addition of AMS to the spray tank
reduces the amount of glyphosate inactivated by antagonistic salts present in the water.The rate of AMS required to achieve this benefit
is dependant upon the hardness of the water, and can be determined by water testing.Most applicators in Iowa choose to estimate how
much AMS is needed, rather than having their water source tested for hardness.Few water sources in Iowa have sufficient hardness
to require the maximum rate of 17 lb AMS per 100 gal water of AMS recommended on most
glyphosate labels; in the absence of testing, 8.5 lbs per 100 gal of water should be
adequate to counteract the antagonistic effects of most water sources in Iowa.

In addition to negating the effects of hard
water, AMS may enhance glyphosate performance on velvetleaf regardless of water quality.The leaf surface of velvetleaf has relatively high
concentrations of calcium salts.This
calcium on the leaf surface may antagonize glyphosate in the same manner as the salts in
hard water.AMS reduces the formation of
calcium-glyphosate complexes on velvetleaf leaves and therefore improves performance.

The Roundup UltraMax label recommends the use
of3 to 30 gallons of water per acre, whereas
the Touchdown IQ label suggests a volume of 3 to 40 gallons.Research has documented increased performance of
glyphosate when applied in water volumes below 10 GPA compared to 20 GPA or higher.There are two primary factors responsible for this
response.First, as spray gallonage
increases, the quantity of antagonistic salts increases.Thus, the potential for calcium or magnesium salts to inactivate glyphosate
increases as spray volume increases.

The second factor that may cause reduced
glyphosate performance at high spray volumes is a simple dilution effect.As spray volume increases, the ratio of formulated
glyphosate to water decreases (one quart of Roundup in 10 gallons water = 1:40; one quart
of Roundup in 20 gallons = 1:80).The ratio
of active ingredient to water is probably of little significance, but as spray volume
increases the amount of surfactant per gallon of water also decreases.The decrease in surfactant concentration that
occurs as spray volume increases may negatively impact product performance under certain
situations.

Several factors should be considered when
selecting a spray volume to use with glyphosate, including effects on herbicide activity,
target coverage and drift potential.Two
potential problems with low spray volumes are increased risk of drift and less effective
penetration and coverage of dense plant canopies.Relatively
small spray droplets are required to achieve uniform coverage at spray volumes less than
10 GPA.While small droplets can increase
glyphosate activity, they may increase the likelihood of spray drift.The second limitation to low spray volumes is that
spray coverage of the target may be diminished.As
spray volume is reduced there is an increased variability in deposition of spray droplets
and thus a greater likelihood that individual weeds may not intercept a lethal dose of the
herbicide.The variability in spray
deposition increases as the density of the plant canopy increases. For most
agronomic situations, 10 to 15 gallons per acre has been shown to minimize deleterious
effects on performance while allowing effective coverage of weeds present in corn and
soybeans.Higher volumes (20 GPA) may be
beneficial in situations with dense weed infestations, well-developed crop canopies, or
large weeds.

Managing environmental-induced fluctuations
in herbicide efficacy is one of the most difficult challenges of persons involved in weed
control.Attempts to develop tools to
aid farmers or custom applicators in determining the optimum herbicide rate or spray
additive based on prevailing weather conditions have been hindered by the complex
interactions between plants and the environment.In
one study, researchers searched for the key environmental factors that affected
postemergence herbicide performancein 60 research trials.The
effectiveness of the herbicide was strongly affected by minimum temperatures in the seven
days prior to application,soil moisture deficits during the
ten days prior to application, and the maximum temperature on the day of application.The task of adjusting application
parameters in response to the environment is further complicated by the fact that each
weed species responds differently to the environment.Thus, a single decision guide for adjusting spray parameters in response to
weather would have limited applicability because of the mixed weed infestations found in
most fields.

Our
limited understanding of how weeds adapt to environmental fluctuations restricts how we
can use weather information to optimize glyphosate applications.However, the likelihood of performance failures
can be reduced by monitoring weather conditions and adjusting application parameters
accordingly. While we are unable to predict the precise herbicide rate needed under
specific conditions, we can predict when weeds are less susceptible to control.
Under these conditions, herbicides rates should be increased or applications delayed until
more favorable conditions occur.

Time of daySoon after the introduction of Roundup Ready
soybeans, control problems with glyphosate applications made in the evening were observed.Subsequent research confirmed that the activity of
glyphosate can decline with applications made early in the morning or in the evening.In certain weed species, this response is at least
partially due to diurnal leaf movements.Leaves
of velvetleaf and many other plants hang vertically after the sun has set, and then raise
parallel to the soil surface during the day in order to intercept sunlight efficiently.Changes in leaf orientation can influence how much
herbicide spray is intercepted by a weed.

Researchers in Arkansas evaluated the
influence of time of glyphosate application on several weed species (Table 3).Both hemp sesbania and sicklepod expressed diurnal
leaf movements, with leaves oriented approximately 10 degrees from horizontal during the
day and at 80 degrees during the night and early morning.Approximately 70% less herbicide was intercepted by the weeds when applications
were made at night than during the day due to the change in leaf orientation.Control of hemp sesbania was closely correlated
with leaf orientation, with control below 50% when applications were made during in the
morning or evening, compared to 80% control when applied at 11 AM.Although sicklepod control varied with time of
application, Roundup performance was not as closely correlated with leaf orientation as
seen on hemp sesbania.

The
research indicated that a decrease in spray interception due to leaf orientation may be
responsible for performance problems when Roundup was applied late in the day or early in
the morning. However, the research also shows that other factors were involved in
this response.Many physiological processes
in plants are influenced by light, and it is possible that changes in plant metabolic
activity between the dark and light also influence herbicide activity. Other
researchreported that the amount of
glyphosate required to reduce the activity of the target site enzyme (EPSPS) was more than
two times greater in the dark than in the light (Tokhver,-A.K.; Pal'm,-E.V. Light-dependence of the
inhibiting action of glyphosate on the shikimate pathway in cotyledon leaves of buckwheat
seedlings. Sov-Plant-Physiol. 33: 748-753.).

Problems with reduced control when
glyphosate is applied in the evening or morning are most likely to occur with species that
have a relatively high level of tolerance to the herbicide. In some cases the
influence of time may be overcome by increasing the rate of herbicide; however, there are
no concrete guidelines to determine when and how much to increase rates to overcome this
affect.

Dew A
wide range of views on the influence of dew on herbicide performance exists among farmers.Some state that they see the best herbicide
performance when a light dew covers the foliage of weeds, whereas others believe the
presence of dew greatly reduces weed control.A
recent study investigated the interaction between dew and spray volume on glyphosate
performance (Table 4).Glyphosate activity
was not affected by dew when applied at the lower spray volumes (16 and 32 GPA).However, control was reduced with 100% dew
coverage on the foliage when glyphosate was applied in 48 GPA.In addition, glyphosate activity was reduced at 48
GPA compared to the lower spray volumes at all dew levels.The researchers speculated that the diminished glyphosate activity at high spray
volume and 100% dew was caused by spray runoff from the saturated leaf surface.They concluded that moderate levels of dew would
have minimal impact on glyphosate when applied at typical spray volumes.

Table 4.Influence of dew and spray volume on glyphosate
control of oats.(Kogan and Zuniga.2001.Weed
Technol. 15:590-593).

As with any herbicide, the performance of glyphosate is
affected by many factors, several which the applicator has little or no control over.
Performance variability due to differences in the formulation used is relatively
small compared to that caused by environmental and application parameters. The
potential for control failures varies widely among weed species based on their inherent
sensitivity to glyphosate. Control of giant foxtail with glyphosate is less likely
to be affected by environmental conditions than velvetleaf simply because giant foxtail is
much more susceptible to glyphosate. Use of too low of rate for the target species
and plant stress caused by environmental conditions are the primary source of control
failures with glyphosate. Keep in mind that any condition that reduces the growth
rate of plants probably will reduce the activity of glyphosate. Under these
conditions, consider all factors that influence herbicide activity in order to minimize
the risk of control failures.

This article was originally prepared for the Iowa Soybean Digest, sponsored by the
Iowa Soybean Promotion Board. It also appears in the Proceedings of the 2001
Integrated Crop Management Conference, Iowa State University.

Common chemical and
trade names are used in this publication. The use of trade names is for clarity by the
reader. Inclusion of a trade name does not imply endorsement of that particular brand of
herbicide and exclusion does not imply nonapproval.